1. Field of the Invention
This invention relates to digital protection and metering apparatus for ac electrical systems, and more particularly, to the manner in which the waveforms of the electrical system are sampled and used to provide protection and metering functions.
2. Background Information
State of the art electrical apparatus for providing protection, and or metering, functions in an ac electrical system typically incorporate microcomputers to perform the various required functions utilizing samples of current, and when needed voltages, in the electrical system. In addition to performing the necessary protection and metering calculations, the microcomputer also manages sampling of the waveforms to generate the digital information needed by the microcomputer. Sampling of the currents and voltages, especially in a three-phase system, occupies considerable time of the microcomputer. Thus, there are competing demands placed upon the microcomputer.
The problem is complicated by the fact that the sampling requirements of the different functions are different. For example, the current sampling or measurement requirements for the protective functions such as instantaneous, short time delay and long time delay are not only different among themselves, but also can be significantly different from those required for metering. The greatest difference is between the sampling requirements for instantaneous overcurrent protection and for metering.
Instantaneous protection implies no intentional delay between the time that the instantaneous trip level setting is exceeded and the time the trip unit opens the breaker. By necessity, this requires that only a few data points can be used to calculate the value of the current as the sampling and the ensuing digital calculations take time which must be limited if the trip is to occur in less than one cycle of the power line.
Conversely, the metering calculations do not need to be performed rapidly as time is not an issue. For instance, if the information is to be displayed on the unit, a display refresher update time of less than 250 ms is undesirable as a faster rate could make the digital display unreadable. Furthermore, if the current being measured is rich in harmonics, it is desirable to use a large number of sample points. All this might imply that the sampling rate suitable for performing the instantaneous protection function would also be suitable for the metering function by merely waiting until the large number of samples were gathered over several cycles. However, samples repetitively taken at the same point in the waveform during successive cycles do not provide the information needed for metering. The large number of samples must be spread over a cycle of the waveform in order to capture the harmonic content needed for accurate metering. On the other hand, if the sampling rate per cycle is made high enough to satisfy the metering requirements, the typical microcomputer which can be used economically in such apparatus cannot perform the required calculations on a real time basis.
One approach to this problem has been the development of equivalent sampling techniques. In an equivalent sampling technique, samples are taken at a selected number of samples per cycle which allows the microcomputer sufficient time to manage the sampling and perform its calculations, but sampling is delayed a fraction of a cycle between each repetition of the selected number of samples so that successive sets of samples are taken at different points in the waveform. Samples gathered over several repetitions are treated as having occurred in a single "equivalent cycle," thereby providing a higher "equivalent sampling rate." For example, where a waveform is sampled at sixteen samples per cycle with a "bump" or delay of 1/64 of a cycle between repetitions, an equivalent sampling rate of sixty-four samples per cycle will be generated after four repetitions. Actually, this will occur in 4+1/16 cycles rather than 4 cycles of the waveform; however, this is of no major significance in providing protection and metering functions.
It is known that the RMS value of a pure sinusoidal waveform can be determined from just two samples taken 90 electrical degrees apart, as discussed for example in U.S. Pat. No. 5,060,166. It is also known that large fault currents are limited by the source impedance of the power system, not by the load. The combination of a sinusoidal voltage producing generator and linear line impedance, resistive and inductive, yields nearly sinusoidal fault currents. Thus, two samples taken 90.degree. apart can be used to provide instantaneous protection. However, samples taken 90 electrical degrees apart (relative to the fundamental frequency) do not capture the harmonics of the waveform which are needed for metering, and, in fact, for the long delay protection function also.
There is a need therefore for an improved protection and metering apparatus for electrical systems with a digital sampling technique which better meets the competing requirements of the various functions performed by the apparatus.
There is a related need for such a protection and metering apparatus having a sampling technique which can be performed by affordable microcomputers while leaving sufficient capacity for the microcomputer to perform all the calculations required for comprehensive protection and metering.